(1) Field of the Invention
The present invention relates to an organic electroluminescence (EL) display device, and more particularly to a technique suitable for obtaining satisfactory images even in the presence of pixels poor in light emitting efficiency, such as defective pixels occurring on the leak path of the organic EL layer due to the invasion of foreign matter or the like.
(2) Description of the Related Art
Flat panel type display devices already in practical use or under research for commercialization include liquid crystal displays (LCD), plasma display panels (PDP), field emission displays (FED) and organic EL displays (OLED). Of these display devices, organic EL displays are very promising display devices as typical thin, light-weight self light-emitting display devices. There are two different types of organic EL display devices, including the so-called bottom emission type and top emission type. Incidentally, though the invention will be described below with reference to an active matrix type organic EL display device, its light emitting structure can also be applied to other organic EL display devices including the simple matrix type.
In an organic EL display device, organic EL light emitting layers, each of which emits light in a prescribed color, are stacked over one (lower electrode) of electrodes formed for each pixel over the inner face of an insulating substrate, such as a glass substrate, and the other electrode (upper electrode) is formed over that lower electrode. By applying a voltage between the lower electrode and the upper electrode to inject holes and carriers into the organic EL light emitting layers, the pixels are caused to emit lights of prescribed frequencies. These pixels are two-dimensionally arranged to display an image. Such a display device is disclosed in Japanese Patent Application Laid-Open Publication No. 2003-122301 for instance. Japanese Patent Application Laid-Open Publication No. 2003-122301 discloses an organic EL display device which is provided with satisfactory moving picture displaying characteristics by controlling the luminescence per frame of the image with display data.
FIGS. 5A and 5B illustrate a one-pixel driving circuit of the organic EL display device according to the related art cited above. FIG. 5A is a diagram of the one pixel circuit, and FIG. 5B illustrates the electrode of an organic EL element (OLED). In FIG. 5A, DTL denotes a signal line; RSL, a reset line (scanning line); PWL, a power supply line; and SWL, a light on/off switching signal line. The gate electrode of a first thin film transistor TFT1 is connected to the signal line DTL via a pixel capacitance CAP. The drain electrode of the first thin film transistor TFT1, also referred to as the drive transistor, is connected to the power supply line PWL and its source electrode, to a first electrode of the organic EL element OLED through the drain-source of a second thin film transistor TFT2.
A third thin film transistor TFT3, connected between the connection point between the first thin film transistor TFT1 and the pixel capacitance CAP and the drain electrode of the first thin film transistor TFT1, discharges the accumulated electric charge of the pixel capacitance CAP at the end of one frame period to prepare for the next signal.
The electrode configuration of the organic EL element shown in FIG. 5A is shown in FIG. 5B. The organic EL element is a diode, and a first electrode BEL is an anode for instance, also referred to as the lower electrode (pixel electrode). A second electrode UEL is a cathode for instance, also referred to as the upper electrode (beta electrode). An organic EL light emitting layer is disposed between these first electrode BEL and second electrode UEL.
FIGS. 6A to 6C illustrate a pixel defect that will arise when a leak occurs in a pixel in the organic EL display device of the configuration shown in FIGS. 5A and 5B. FIG. 6A is an enlarged view of the driving circuit for the one pixel shown in FIGS. 5A and 5B, and FIG. 6B is an enlarged view of a pixel part PXC surrounded by broken lines in FIG. 6A. The lower electrode for the pixel is driven by a second thin film transistor TFT2. FIG. 6C shows a state in which a leak path is formed between the lower electrode and the upper electrode and the whole face of the pixel has become unable to emit light (black point defect).